Where is Ruthlessness in the Rogue talent tree?

Assassination tree, row 2, column 1. You need 5 points spent in Assassination before you can learn it.

Is Ruthlessness new in WoW Forever?

No. Ruthlessness works the same as in Classic 1.12, where it was in Assassination row 2 with 3 ranks.

Rank	Effect	Source
1	Gives your finishing moves a 20% chance to add a Combo Point to your target.	client
2	Gives your finishing moves a 40% chance to add a Combo Point to your target.	client
3	Gives your finishing moves a 60% chance to add a Combo Point to your target.	client

Tree: Assassination, row 2, column 1. Needs 5 points in Assassination first. The earliest you can finish all 3 ranks is level 17 if you put every point into this tree.
Competes with: Improved Expose Armor, Improved Slice and Dice in the same row.

	Classic 1.12	WoW Forever
Name	Ruthlessness	Ruthlessness
Position	Assassination row 2, col 1	Assassination row 2, col 1
Ranks	3	3
Rank 1 text	Gives your finishing moves a 20% chance to add a combo point to your target.	Gives your finishing moves a 20% chance to add a Combo Point to your target.

Combat Rogue leveling build (10–60) · 3/3
Combat Rogue endgame PvE build (18/33/0) · 3/3
Assassination Rogue endgame PvE build (31/20/0) · 3/3
Subtlety Rogue PvP build (20/0/31) · 3/3
Combat Rogue PvP build (16/35/0) · 3/3